Instrument transformer test equipment and method

ABSTRACT

A method and apparatus for testing a transformer. A controller generates a switching signal and supplies the switching signal to a solid state switch in circuit with a test load of known magnitude, switching the solid state switch to place the test load in circuit with the transformer. A signal through or across the transformer is then measured. Methods and apparatus for testing a transformer comprise the controller and solid state switch placing a series of test loads of different known magnitudes in circuit with the transformer from the lowest magnitude to the highest, from the highest to the lowest, or both. Additionally, a test load array comprises a plurality of power resistors, a heat sink for accommodating the plurality of power resistors, and a material providing electrical insulation of the power resistors from the heat sink and thermal conductivity between the power resistors and the heat sink.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase of PCT/US2008/072873 filedAug. 12, 2008. PCT/US2008/072873 claims the benefit under 35 U. S. C.§119(e) of U.S. Ser. No. 60/956,322, filed Aug. 16, 2007. The entiredisclosures of U.S. Ser. No. 60/956,322, and PCT/US2008/072873 arehereby incorporated herein by reference.

FIELD OF INVENTION

This invention relates to instrument transformer test equipment. It isdisclosed in the context of equipment and methods for burden testing ofinstrument transformers, but is believed to have utility in otherapplications as well.

BACKGROUND OF THE INVENTION

The prior art includes U.S. Pat. Nos. 7,0303,602; 6,680,616; 6,160,697;Serway, Physics for Scientists and Engineers, Saunders College Pub.1996; and IEEE Standard C57.13-1993 Standard Requirements for InstrumentTransformers. The disclosures of these references are herebyincorporated herein by reference. The above listing is not intended tobe a representation that a complete search of all relevant art has beenmade, or that no more pertinent art than that listed exists, or that thelisted art is material to patentability. Nor should any suchrepresentation be inferred.

SUMMARY OF THE INVENTION

According to an aspect of the invention, a method for testing atransformer comprises operating a controller to generate a switchingsignal, supplying the switching signal to a solid state switch incircuit with a test load of known magnitude, switching the solid stateswitch to place the test load in circuit with the transformer, andmeasuring a signal through or across the transformer.

Illustratively, the method further includes analog-to-digital convertingthe measured signal.

Illustratively, the method further includes locking the phase of theswitching signal to the phase of the measured signal.

Illustratively according to the method, supplying the switching signalto the solid state switch in circuit with the test load comprisessupplying the switching signal to the solid state switch in parallelwith the test load.

Illustratively according to the method, supplying the switching signalto the solid state switch in circuit with the test load comprisessupplying the switching signal to the solid state switch in series withthe test load.

Illustratively according to the method, supplying the switching signalto the solid state switch in circuit with the test load comprisessupplying the switching signal to the solid state switch inseries-parallel with the test load.

Illustratively, the method further includes monitoring zero crossings ofthe measured signal.

Illustratively, the method further includes operating the controller togenerate the switching signal substantially at the zero crossings of themeasured signal.

Illustratively, the method further includes analog-to-digital convertingthe measured signal.

Illustratively according to the method, supplying the switching signalto a solid state switch comprises supplying the switching signal to afield effect transistor.

According to another aspect of the invention, apparatus for testing atransformer comprises a controller for generating a switching signal, atest load of known magnitude, and a solid state switch in circuit withthe test load. The solid state switch has a control port coupled to thecontroller for supplying the switching signal to the solid state switchto place the test load in circuit with the transformer. The apparatusfurther comprises a measurement device coupled to the test load formeasuring a signal through or across the transformer.

Illustratively, the apparatus further includes an analog-to-digitalconverter coupled to the transformer and to the measurement device foranalog-to-digital converting the measured signal.

Illustratively according to this aspect, the controller is coupled tothe test load to lock the phase of the switching signal to the phase ofthe measured signal.

Illustratively according to this aspect, the solid state switch iscoupled in series circuit with the test load.

Illustratively according to this aspect, the solid state switch iscoupled in parallel circuit with the test load.

Illustratively, the apparatus further includes multiple solid stateswitches. The solid state switches are coupled in series-parallelcircuit with the test load.

Illustratively according to this aspect, the controller is coupled tothe test load to monitoring zero crossings of the measured signal.

Illustratively according to this aspect, the controller supplies theswitching signal to the solid state switch substantially at the zerocrossings of the measured signal.

Illustratively, the apparatus further includes an analog-to-digitalconverter coupled to the transformer and to the measurement device foranalog-to-digital converting the measured signal.

Illustratively according to this aspect, the solid state switchcomprises a field effect transistor.

According to another aspect of the invention, a method for testing atransformer comprises placing a series of test loads of different knownmagnitudes in circuit with the transformer from the lowest magnitude tothe highest.

Illustratively, the method further includes placing the series of testloads of different known magnitudes in circuit with the transformer fromthe highest magnitude to the lowest.

According to another aspect of the invention, a method for testing atransformer comprises placing a series of test loads of different knownmagnitudes in circuit with the transformer from the highest magnitude tothe lowest.

According to another aspect of the invention, apparatus for testing atransformer comprises a series of test loads of different knownmagnitudes ranging from a lowest magnitude to a highest magnitude. Theapparatus includes multiple solid state switches coupled to the testloads of the series for placing the test loads of the series in circuitwith the transformer and a controller for controlling the multiple solidstate switches first to place the series of test loads in circuit withthe transformer from the lowest magnitude to the highest.

Illustratively according to this aspect, the controller for controllingthe multiple solid state switches first to place the series of testloads in circuit with the transformer from the lowest magnitude to thehighest further controls the multiple solid state switches to then placethe series of test loads in circuit with the transformer from thehighest magnitude to the lowest.

According to another aspect of the invention, apparatus for testing atransformer comprises a series of test loads of different knownmagnitudes ranging from a lowest magnitude to a highest magnitude. Theapparatus includes multiple solid state switches coupled to the testloads of the series for placing the test loads of the series in circuitwith the transformer and a controller for controlling the multiple solidstate switches first to place the series of test loads in circuit withthe transformer from the highest magnitude to the lowest.

According to another aspect of the invention, a test load arraycomprises a plurality of power resistors. Each power resistor has a pairof terminals for selectively coupling to a device to be tested. The testload array further comprises a heat sink for accommodating the pluralityof power resistors and a material for providing electrical insulation ofthe power resistors from the heat sink and thermal conductivity betweenthe power resistors and the heat sink.

Illustratively, the apparatus further includes a fan for moving airacross the heat sink. The fan mounted on the heat sink.

Illustratively, the apparatus further includes a temperature sensormounted on the heat sink. The temperature sensor controls the fan.

Illustratively, the apparatus further includes a controller. Thetemperature sensor is coupled to the controller to provide a signalrelated to the temperature of the heat sink to the controller. Thecontroller is coupled to the fan to control operation of the fan.

BRIEF DESCRIPTIONS OF THE DRAWINGS

The invention may best be understood by referring to the followingdetailed descriptions and accompanying drawings which illustrate theinvention. In the drawings:

FIG. 1 illustrates diagrammatically an installation at a transformersubstation;

FIG. 2 illustrates diagrammatically circuits for performing current andvoltage burden tests;

FIG. 3a-e illustrate diagrammatically a high accuracy, high speedmeasurement circuit;

FIG. 4 illustrates the burden switching cycle required for single cyclemeasurements that that reduce the likelihood of certain problems relatedto residual flux in the core of a transformer under test;

FIGS. 5a-b illustrate parallel switching elements with a seriesconnected burden;

FIGS. 6a-b illustrate series switching elements with a parallelconnected burden;

FIG. 7 illustrates a perspective view of a power resistor arrayintegrated into a common heat sink;

FIG. 8 illustrates a perspective view of a prior art power resistorhoused in a heat sink;

FIGS. 9a-d illustrate diagrammatically a transformer under test, aburden tester and associated protection and control circuitry;

FIG. 10a illustrates a schematic of the power, low current path, andcommunication interface and pass through;

FIG. 10b illustrates a schematic of drive circuit for a solid stateswitch;

FIG. 10c illustrates a schematic of a temperature sensor;

FIG. 10d illustrates a schematic of an LED drive circuit; and,

FIGS. 11a-d illustrate a flow diagram of a program for controlling theapparatus illustrated in FIGS. 9a-d and 10a -d.

DETAILED DESCRIPTIONS OF ILLUSTRATIVE EMBODIMENTS

In the descriptions that follow, several integrated circuits and othercomponents are identified, with particular circuit types and sources. Inmany cases, component values, terminal names and pin numbers for thesespecifically identified circuits and components and sources are noted.This should not be interpreted to mean that the identified circuits andcomponents are the only circuits and components available from the same,or any other, sources that will perform the described functions. Othercircuits and components are typically available from the same, andother, sources which will perform the described functions. The terminalnames and pin numbers of such other circuits and components may or maynot be the same as those indicated for the specific circuits andcomponents identified in this application.

Power distribution from the generating plant to the load is accomplishedby high-voltage, high-current transmission lines. The high voltages (forexample, 345 KV or 765 KV) and currents (for example, in the thousandsof amperes) necessary to transmit power efficiently from the plant areimpractical for ordinary use at most loads. Generally, substationsconvert input voltage levels to output voltage levels (14.4 KV, 440 V,and so on) that are better suited for use at subscribers' loads. Asubstation may also interconnect multiple power generators' networksinto a grid. The or each power generator typically monitors itscontribution to the energy at the substation or interconnection for, forexample, billing purposes. The voltages and currents at the substationsare typically too large to be measured directly by an energy meter.Instead, an instrument transformer generally is used. A potential orvoltage transformer (hereinafter sometimes VT) and a current transformer(hereinafter sometimes CT) are employed to generate precision ratiovoltages and currents, and thus precision ratios to the energy passingthrough the substation at levels that are compatible with the energymeters installed at the substation; (typically 69 to 600 V AC and 0-20 AAC). FIG. 1 is a diagrammatic illustration of such an installation. Foreconomic reasons, it is important to measure accurately the energypassing through a substation. One of the ways in which instrumenttransformers are rated is in accordance with so-called “Accuracy Classat Standard Burden” as defined in IEEE Standard C57.13-1993. Periodictesting may be required to verify that an instrument transformermaintains rated accuracy at rated burden while in service. Since the1960s, various test equipment has been devised to test instrumenttransformers in service. FIG. 2 illustrates diagrammatically a basicsetup for burden testing. Such a test sequence may proceed generally asfollows: a voltage or current reading M1 is measured. The rated burdenis inserted into the circuit (generally in parallel for voltage andgenerally in series for current) and a second measurement M2 is taken.The burden is then removed from the circuit. The two readings are thencompared as shown in equation 1 to determine the accuracy of thetransformer.(M1−M2)/M1×100%≦accuracy Class×100%  eq. 1

There are several problems associated with in-service testing ofinstrument transformers. First, burden testing by its nature assumesthat the primary current is constant during the test interval. Smallload shifts during a test yield inaccurate results. Further, internalheating of the burden may contribute to inaccuracies. As an example, an8Ω load conducting 5 A rms needs to be able to dissipate 200 W. However,temperature coefficients are associated with all resistors. The longercurrent passes through the burden, the more the burden heats up. Themore the burden heats up, the greater is its drift from its nominalresistance.

Additionally, in currently available burden testers, the operator cannotcontrol the points in the AC cycle at which the burden is inserted andremoved. Adding or removing a burden when the current is at or near itspeak can result in transformer magnetization. Transformer magnetizationcan result in flux offset in the core of the transformer under test(hereinafter sometimes TUT). This remnant offset flux can reduce therange of the transformer and result in an inaccurate output signal.

Still further, the length of time that the burden needs to be in thecircuit is a function of the settling time of the measurement device andhow quickly the operator can record the measurement before removing theburden. Furthermore, the resistors have definite life spans which areaffected by temperature cycling. This requires that the burden resistorshave power rating capability to dissipate the heat generated duringburden testing. Thus, burden resistors typically are relatively heavy,unwieldy devices. Many early burden testers were designed for “pulse”dissipation operation, and were subject to accidental overload bycareless operators who left burdens in circuit for too long. Whileburning open a burden resistor used for PT testing only leads toinaccurate test results, burning open a burden resistor used for CTtesting can damage the CT and/or the test equipment, and potentiallyharm the operator.

In order to compensate for variations in the primary current during atest interval, the operator may take multiple measurements and averagethe results in an effort to reduce the effect of a varying primary load.However, this compounds the problems associated with internal heating ofthe burden and the length of time that the burden needs to be in thecircuit.

To shorten the time that the burden needs to be in the circuit, theoperator may insert the burden just long enough to read the measurementdevice. This may shorten the time that the burden is in the circuit.However, this technique relies on, for example, the operator's memoryand, as a result, is subject to error. This technique also risks thatthe operator may not allow sufficient settling time for the measurementdevice.

In an effort to minimize these sources of error and to improve safety,automated burden testers were created in the 1980's and 1990's. Theburden control is moved to an on-board microprocessor that automaticallyregulated both the magnitude of the burden and the length of time theburden is in the circuit. Measurements before and after insertion of theburden into the circuit are automatically recorded and the percentagechange is automatically calculated. The percentage change can then becompared with the accuracy class of the transformer under test.Additional features such as data collection and computer downloadcapability also appear in many of these automated burden testers.

The 1980's-1990's generation of burden testers, although an improvementover the “manual” burden test kits, typically requires an array ofcostly, high current relays and a number of discrete power burdenresistors mounted on a relatively massive heat sink. The length of timeany particular burden is in circuit depends on the switching speeds ofthe associated relays (which can be greater than 10 msec) and thesettling time of the measurement circuit. Also, the 1980's-1990'sgeneration of transformer testers, using relays, does not addresscontrol of the points in the waveform at which the burden is insertedinto, and removed from, the measurement circuit. This may result in aburden being added and/or removed in mid-cycle. Due to the propagationdelay in the action of the relays and the possibilities of entering andleaving the circuit mid-cycle, two complete cycles ( 1/30^(th) of asecond, or 33.33 msec at 60 Hz) have to be assumed for error,establishing a minimum measurement interval to compensate for insertionvariation. Practically speaking, the prior art contains examples thatrequire a minimum of one full second to take one burden measurement.Additionally, a load change during that one second period may still notavoid inaccuracies in the measurement, and the TUT's core may experiencesignificant remnant magnetization as a result of the test.

In a typical installation illustrated in FIG. 1, a test switch 20 isprovided in the paths 22-1, 22-2, 22-3 of the three phases φ1, φ2, φ3,respectively, of the current transformers 24-1, 24-2, 24-3,respectively, so that a test paddle or “duck bill” 25 (see FIG. 9d ),such as, for example, a Tesco model 1077 make-before-break test plug,may be inserted into the respective current loops without breaking therespective current paths.

Current burdens 26 (see FIG. 2) are added in series with the transformer24-m, 1≦m≦3, secondaries 24-m-s by, for example, opening respectivebypass switches 28-m that otherwise shunt the current. The resistancevalues of the current burdens 26 are dictated by IEEE StandardC57.13-1993, but values in the range of 0.2Ω are typical. A currentmeasurement device, in the illustrated embodiment an ammeter A, isinserted in series with the burden 26-m-switch 28-m parallel combinationto measure the effect of inserting the burden 26-m in the secondary24-m-s of the current transformer 24-m under test. A voltage transformer30-m, 1 may be tested in a similar manner by, for example, placing aburden 32 in parallel with the secondary 30-m-s of the transformer 30-mand closing the respective series switch 36-m, FIG. 2. Again, theresistance values of the voltage burdens 32 are dictated by IEEEStandard C57.13-1993, but values in the range of 288Ω are typical. Avoltage measurement device, in the illustrated embodiment a voltmeter V,is inserted in parallel with the burden 32-m-switch 36-m seriescombination to measure the effect of inserting the burden 32-m in thesecondary 30-m-s of the voltage transformer 30-m under test.

Referring now to FIGS. 3a-e , a high accuracy, high speed measurementcircuit 40 is used to measure the voltage or current of a TUT 24, 30.The accuracy and speed of the circuit 40 are such that it is possible tolock it in phase with the zero crossings 42, FIG. 4, of the voltage orcurrent waveform and perform an rms measurement in a single cycle. Aspreviously noted, prior art high current relays may have on the order of10 msec of lag time while engaging or disengaging, which distorts themeasurement. An appropriate number of cycles must then be integrated inan effort to reduce this distortion. In this disclosure, the prior art'srelays and switches have been replaced by high speed MOSFET switches 28,36 which are capable of switching a burden 26, 32 in under 100 nsec.Under the control of a computer (hereinafter sometimes microprocessor orμP) 44, FIG. 3a , working in conjunction with a digital signal processor(hereinafter sometimes DSP) 46, FIG. 3b , insertion and removal of aburden 26, 32 can be synchronized with the zero crossings 42, FIG. 4, ofthe signal. A more accurate, more distortion-free measurement of theinstrument transformer 24 or 30's response to the burden 26, 32 can becompleted in as short a time as one cycle (16.66 msec at 60 Hz). μPillustratively is an NXP LPC2141 32 bit ARM7 processor. DSP 46illustratively is a Texas Instruments type TMS320 DSP.

The measurement circuit includes the μP 44, the DSP 46, an integratedpower resistor array (hereinafter sometimes IPRA) 50, FIG. 3c ,providing the burden 26 or 32, an analog-to-digital converter(hereinafter sometimes ADC) 53 and compensated transformer 59, FIG. 3d ,and a power supply 55 and user interface 57, FIG. 3e , all coupled incircuit as illustrated in FIGS. 3a-e with the transformer CT or VT undertest. Illustratively, ADC 53 is of the general type described in U.S.Pat. No. 6,833,803, the disclosure of which is hereby incorporatedherein by reference. Illustratively, compensated transformer 59 is ofthe general type described in U.S. Pat. No. 5,276,394, the disclosure ofwhich is hereby incorporated herein by reference.

Power supply 55 (FIG. 3e ) receives at an input port AC LINE IN[0:1]power from the substation or the like at substation level, for example,60 to 600 VAC, and provides at an output port POWER[0:1] the necessaryvoltages and currents to power all of μP 44 (FIG. 3a ), DSP 46 (FIG. 3b), IPRA 50 (FIG. 3c ), compensated transformer 59 and ADC 53 (FIG. 3d )to all of which it is coupled either directly, or, in the case of IPRA50, from the μP 44's POWER2[0:1] port to the IPRA 50's POWER[0:1] port.

The user interface 57, FIG. 3e , which may be a stand-alone PC connectedor coupled wirelessly (e.g., by WiFi or Bluetooth protocol) or may beincorporated into the measurement device 62, includes user port USERINPUT[0:2], which may include KEYBOARD/TOUCHSCREEN, MOUSE/TOUCHPAD ANDMONITOR/GUI I/Os, and port SERIAL_PC[0:1] which is coupled to DSP 46'sSERIAL_PC[0:1] port. DSP 46's SERIAL [0:1] port is coupled to μP 44'sSERIAL [0:1] port. μP 44's BURDEN CONTROL [0:19] port, TRANSFORMERBYPASS RELAY CONTROL port, BURDEN BYPASS RELAY CONTROL port, TEMPERATUREMONITOR port, OVERBURDEN MONITOR port and FAN CONTROL port are coupled,respectively, to IPRA 50's BURDEN CONTROL [0:19] port, TRANSFORMERBYPASS RELAY CONTROL port, BURDEN BYPASS RELAY CONTROL port, TEMPERATUREMONITOR port, OVERBURDEN MONITOR port and FAN CONTROL port. IPRA 50'sTRANSFORMER IN[0:1] port is coupled to the transformer CT or VT undertest. IPRA 50's TRANSFORMER OUT[0:1] port is coupled to the measurementcompensated transformer 59's TRANSFORMER IN[0:1] port. The measurementcompensated transformer 59's ANALOG SIGNAL [0:1] port is coupled to theADC 53's ANALOG SIGNAL [0:1] port.

Referring to FIGS. 5a-b and 6a-b , the MOSFET switches 28, 36 can beplaced in parallel with the burden elements 26, 32 (FIGS. 5a-b ,particularly FIG. 5b ), in series with the burden elements 26, 32 (FIGS.6a-b , particularly FIG. 6b ) or in any combination of series andparallel required to achieve the desired composite burden 26, 32value(s). The number n of stages of switching that is desirable dependsupon the desired range and increments of burdens to be added to the testloop. In an example, n might equal 10, with one resistor 26 n of 0.2Ω,one of ⅓Ω, seven resistors of 1Ω resistance each, and one of 2Ωresistance. This permits a range of resistance values, including desiredseries combinations of the various resistor 26-n values to be insertedinto the current loop of the secondary of the current transformer undertest. The presence or absence of any resistor 26-n in the current loopis determined by the status of its respective switch 28-n. In a similarmanner, a range of resistance values 32-n including desired parallelcombinations of the various resistance values 32-n can be inserted intothe voltage loop of the secondary of the voltage transformer under test.Switches 26-n each illustratively comprise two International RectifierIRF6618 FETs. Higher voltage FETs are required for switches 36-n.Typical turns ratios for current transformers 24-m are 400:5, providinga 5 A secondary current for a 400 A primary current, 200:5, providing a5 A secondary current for a 200 A primary current, 100:5, providing a 5A secondary current for a 100 A primary current, and so on. In anillustrative circuit constructed in accordance with FIGS. 5a-b , tenburden resistors 26-1, 26-2, . . . 26-10 having values of 0.2Ω, ⅓Ω, 1Ω,1Ω, 1Ω, 1Ω, 1Ω, 1Ω, 1Ω, and 2Ω. These values permit a number ofdesirable current burdens to be put in circuit with the secondary 24-m-sof current transformer 24-m.

With the availability of single cycle measurements and flexible highspeed switching, a test cycle may, for example, proceed as illustratedin FIG. 4, with the signal being applied sequentially to a series ofburdens 26, 32 incrementing from the highest value to the lowest value.An illustrative test cycle might then continue from the lowest valueback to the highest. Such a test cycle can measure the response of aninstrument transformer 24, 30 at, for example, seven burden 26-n, 32-nvalues in less than 0.5 second at 60 Hz. The length of such a test cyclemay be calculated from:Time=(Nburdens×Cycles/Burden)/Freq  eq. 2where Nburdens is the number of burdens 26-n or 32-n to be switchedthrough, Cycles/Burden is the number of cycles during which each burdenis in circuit during each step of the test, Freq is the source frequency(60 Hz in the illustrated example), and Time is the length of themeasurement cycle.

In addition, such a test cycle can effectively demagnetize a core thatmight otherwise be operating with a residual flux by subjecting it toprogressively lower induced magnetic field, effectively ramping downthrough the hysteresis curve.

The energy that must be dissipated by the burden 26-n, 32-n is directlyproportional to the applied power and the time the burden 26-n, 32-n isin circuit. As examples, consider the amounts of heat that must bedissipated by an 8 ohm burden 26, 32 with an applied 20 A rms signal for0.8 seconds and for 16.66 msec.8Ω×(20 A)²×0.8 sec=2560 joules  eq. 38Ω×(20 A)²×0.01666 sec=53 joules  eq. 4The temperature change, ΔT, is directly proportional to the thermalenergy, Q.ΔT=Q/mc  eq. 5where m is the mass of the material being heated and c is the specificheat of the material. Thus, for two devices of equal mass m and specificheat c, the described method of burden testing results in a temperaturechange about one fiftieth (53 joules/2560 joules=0.0207) thatexperienced with prior art methods. A one and a half orders of magnitudedrop in test time equates to a one and a half orders of magnitude dropin energy that needs to be dissipated.

Referring now to FIGS. 7 and 8, a feature of prior art discrete powerresistors was used to advantage. Typically, only one or a small subsetof the total number of available burden resistors 26-n, 32-n are used ata time. According to this aspect, then, the heat sink(s) housing all ofthe resistors 26-n, 32-n can be used to help dissipate the heat whenonly one or two resistors may be used during a test, effectivelyincreasing m in equation 5 above. The multiple burden resistors 26-n,32-n are mounted together in IPRA 50. Discrete high power handlingcapacity resistors 26-1, 26-2, . . . 26-n, 32-1, 32-2, . . . 32-n arepotted into a common heat sink 52 constructed from a material, such as,for example, aluminum, copper or the like, having a high heat capacity,using a highly thermally conductive, yet electrically insulating pottingmaterial 51, thereby forming an aggregate or composite burden element.The illustrative resistors 26-1, 26-2, . . . 26-n, 32-1, 32-2, . . .32-n are ten in number (n=10), and are generally circular in crosssection transverse to their longitudinal extents. An illustrativepotting material 51 is Aremco Products Inc, CERAMACAST 675 mixed withgreen silicon carbide at a ratio of 5:3. Generally circular crosssection channels are provided in heat sink 52 for slidably receiving tworesistors 26-1, 26-2, . . . 26-n, 32-1, 32-2, . . . 32-n each. Theresistors 26-1, 26-2, . . . 26-n, 32-1, 32-2, . . . 32-n are coatingwith the potting material 51 before or as they are inserted into thechannels, and additional potting material is added to fill the channelsafter the resistors are inserted into the channels. The potting materialis then permitted to harden, fixing the resistors 26-1, 26-2, . . .26-n, 32-1, 32-2, . . . 32-n in thermal contact with the heat sink 52while electrically insulating them from the heat sink 52. When comparedto, for example, ten discrete parts such as, for example, Vishay RH-50resistors, FIG. 8, the IPRA 50 has 30% less weight, and any switchedseries and/or parallel combination of values is capable of dissipating105 W continuously at 25° C. A single unmounted Vishay RH-50 resistor,FIG. 8, is capable of dissipating 12 W when operated in free air.

Referring now to FIGS. 9a-d , a burden tester 54 (FIGS. 3a and 3c )configured for testing an instrument current transformer 56, FIG. 9a ,comprises a test paddle or duck bill 25 having leads 25-1, 25-2 forcoupling to high and low side leads 56-s-1 and 56-s-2, respectively, ofthe secondary 56-s of transformer 56. The high side 25-1 of the duckbill 25 is coupled to a high current connector pin 50-1, FIGS. 9a-b ,that is coupled to the high side of an IPRA 50 in the burden tester 54.The low side 25-2 of the duck bill 25 is coupled via a high currentconnector pin 50-2 to one terminal 70-p-1 of the primary winding 70-p ofa continuity transformer 70 in the burden tester 54. First 60-1 andsecond 60-2 high current paths couple the burden tester 54 to a highaccuracy DSP 46-controlled current measurement device 62 (FIGS. 3b, dand e ). The first and second high current paths 60-1, 60-2,respectively, are coupled together by high-current, low-bias clampingdiodes 64-1, 64-2, FIG. 9c , to provide a high current, low burden pathif the current measurement device 62 is not in circuit. Diodes 64-1,64-2 illustratively are ST Microelectronics STPS30H60CG diodes. Thefirst high current path 60-1 couples the low side of the IPRA 50 to ahigh current test lead 66 terminated with a high current plug 68.

The second high current path 60-2 couples a low side terminal 70-p-2 ofa primary 70-p of continuity transformer 70 to a high current test lead72 terminated with a high current plug 74. The n resistors 26-1, 26-2, .. . 26-n, FIGS. 9b and d , in the IPRA 50 are coupled in series. A relay76, when activated, shunts the secondary 56-s of the current transformer56 by shunting high current connector pin 50-1, FIG. 9a , to highcurrent connector pin 50-2 when relay 76 is closed. An illustrativerelay 76 is the Tyco RTD14012F relay.

A high current fault circuit 78 is placed across high current connectorpin 50-1 and high current connector pin 50-2. Circuit 78 monitors thevoltage across secondary 56-s of the current transformer 56 under testand clamps the secondary 56-s in the event of an over-burden condition,such as, for example, when the voltage across the secondary 56-s getsabove 60 volts peak. Circuit 78 also shorts the secondary until relay 76closes. A typical relay 76 has a 20 msec switching time.

A second relay 80 shunts the IPRA 50 high side 50-h to its low side 50-lwhen closed. An illustrative relay 80 is the Tyco RTD14012F relay.

A high-side terminal 70-s-1, FIG. 9c , of the secondary 70-s of thecontinuity transformer 70 is coupled to an output terminal of anamplifier 84 having an inverting (−) input terminal and a non-inverting(+) input terminal. A low-side terminal 70-s-2 of the continuitytransformer 70 is coupled to ground. The output terminal of theamplifier 84 is coupled through a 1 KΩ resistor to the − input terminalof amplifier 84. The − input terminal of amplifier 84 is further coupledthrough a 1 KΩ resistor to ground. The microprocessor 44 is coupled by ahigh pass filter 86 including a resistor having a resistance of,illustratively, 1 KΩ, and a capacitor having a capacitance of,illustratively, 10 μF, to the + input terminal of the amplifier 84. Asignal may be generated by the microprocessor 44 and coupled to the highcurrent path 60-1, 60-2 and measured by the current measurement device62 to assure that a good path exists before applying a burden 26-1,26-2, . . . 26-n to the current transformer 56 secondary 56-s. Anillustrative continuity transformer 70 has a turns ratio of 125:2. Thehigh side terminal 70-s-1 of secondary 70-s is also coupled through alow-value load resistor, illustratively, a 5 SI resistor, and a relay100 to ground. Relay 100 illustratively is an Omron G5V relay. Thepurpose of relay 100 is to shunt the secondary 70-s when amplifier 84 isnot injecting a signal. Otherwise, an impedance will be reflected backinto the load of the secondary 56-s of the current transformer 56.Illustratively, amplifier 84 is a Burr-Brown (now Texas Instruments)type OPA227, V_(CC) is 6V and V_(EE) is −6V.

A MOSFET switch 28-1, 28-2, . . . 28-n couples the high side of each ofresistors 26-1, 26-2, . . . 26-n, respectively, in the IPRA 50 to therespective resistor's low side. Each resistor 26-1, 26-2, . . . 26-nthus may be individually put in or taken out of circuit by turning itsrespective MOSFET switch 28-1, 28-2, . . . 28-n off or on.

Turning now to FIG. 10a , a low power plug 90 is provided to obtainauxiliary power, for serial communication and interfacing between theburden tester 54, FIGS. 3a and c , and the measurement device 62, FIGS.3b, d and e . A low power jack 96 is provided to “daisy chain”additional devices to auxiliary power, to the serial communication lines98, and for interfacing the low level current measurement circuit 62,FIGS. 3b, d and e . Relay 100, FIGS. 9d and 10a , permits selectionbetween coupling the local continuity circuit, FIG. 9d , or a daisychained device coupled to jack 96 to the current measurement circuit 62,FIGS. 3b, d and e . Relay 100 is driven from μP 44 through a bipolartransistor, illustratively a type 2N2222.

A clamp-on current probe, such as, for example, an AEMC model JM845Aclamp-on current probe, may be attached to the pass-through connector 96so that the primary current at the installation may be compared to thesecondary current being measured directly on the high current path 60-1,60-2.

Microprocessor 44 controls the relays 76, 80, 100 and MOSFET switches28-1, 28-2, . . . 28-n or 36-1, 36-2, . . . 36-n, monitors a temperaturesensor 110, FIG. 10b , affixed to the IPRA 50, monitors the overburdentrip circuit 78, sources the voltage at the output port of amplifier 84for the continuity test, modulates the fan 114, FIG. 7, associated withIPRA 50, and communicates with the DSP 46 over the serial communicationlines 98. The temperature sensor 110 is coupled to the microprocessor 44via two-wire serial line 98. Temperature sensor 110 illustratively is aNXP SA56004 with V_(DD) of 3.3 V.

Each MOSFET switch 28-n, 36-n is driven from the microprocessor 44through a buffer 116-n, driving a pulse transformer 118-n. See FIGS. 5a,6a and 10c . An illustrative buffer 116-n is the Texas InstrumentsUCC27324D FET driver with V_(CC) of 6V, driving a pulse transformer118-n having a turns ratio of 1:2, and a driver transistor such as an ONSemiconductor or Motorola BSS138 FET coupling the pulse transformer tothe input ports of switches 28-n, 36-n.

Light-emitting diodes (LEDs) 122, FIG. 10d , are coupled to themicroprocessor 44 to provide a visual indication of the status of theburden tester 54. Illustratively, LEDs 122 are a Dialight 521-9450.Because the power supply in the illustrated embodiment had limitedpower, the circuit of FIG. 10d was provided to shift the voltage supplylevel from the +3.3 V supply to the μP 44 to the system's −6 V supply.In the circuit illustrated in FIGS. 10d , R53 and R54 are 3.16 KΩresistors, Q42 and Q45 are type 2N2907 bipolar transistors, Q43 and Q44are type BSS138 FETs, R55 and R56 are 46.4 KΩ resistors, and R58 and R59are 316Ω resistors.

Turning now to FIGS. 11a-d , and particularly to FIG. 11a , a userrequest to start a burden test signals the start of a test. The burdenmeasurement logic is switched on, FIGS. 11a and c , the “start of test”trigger is detected by the “count down to start of burden measurement”routine, FIG. 11b , which triggers “start of control.” A vector ofselected burdens to measure is provided to the burden hardwarecontroller, FIGS. 11b and d . A burden counter is incremented, FIG. 11b, and current measurement inputs are provided, FIG. 11c , to a “zerocrossing detection” routine, a “frequency detection” routine and an“integrate and dump” routine. The “zero crossing” routine also providesphase measurement output to an “even zero crossing” decision block, FIG.11c , to the burden hardware (FIGS. 9a-d, 10a-d ) controller, FIGS. 11band d , and to the “count down to start of burden measurement” routine,FIG. 11b . If the zero crossing is not even, FIGS. 11c and b , theburden counter is incremented, FIG. 11b . If the zero crossing is even,FIG. 11c , indicating that a complete cycle of the input current orvoltage has passed, the “integrate and dump” routine dumps theintegrated value to storage and clears the integrator, and the ithburden measurement is stored, FIGS. 11c-d . If the burden counter isincremented, FIG. 11b , the routine asks, FIGS. 11a-b , if the counterhas exceeded the maximum number of burdens 26, 32 with which theinstrument transformer, 24, 30, 56 is to be tested. If not, the routinecontinues to run. If the burden counter has exceeded the maximum numberof burdens, FIG. 11a , the program has reached the “end of test,” andthe routine is switched off.

The illustrated high accuracy current measurement device 62 includes alow burden, precision ratio, current measurement stage 59, FIG. 3d ,coupled to a 24 bit ADC 53 coupled to DSP 46, FIG. 3b . Whileillustrated as embedded or incorporated into the device 40, μP 44 canalso be external to the current measurement apparatus 40 and communicatewith it, for example, via serial communication.

The microprocessor 44's and DSP 46's software configures themicroprocessor 44 and DSP 46 to function together in a phase locked loop(hereinafter sometimes PLL), synchronizing measurements and burdencommands with the zero crossings (see FIG. 4) of the current or voltagebeing measured. The software analyzes the spectrum of the measuredcurrent or voltage, providing harmonic and inter-harmonic information,measures the unburdened current/voltage and the burdened current/voltageand provides the percentage (%) difference measurement. The softwarealso multiplexes between the high current path and the low current path.With a calibrated primary current clamp (not shown) attached to thepass-through connector, the software calculates the ratio of the primarycurrent to the secondary current through, for example, 56-s. Thesoftware further communicates with the built-in user interface 57 of thecurrent measurement device 62 or with an external interface (not shown)by serial communication. This system thus permits the user to interfacewith the transformer test apparatus 40. This includes, but is notlimited to, creating, editing, and executing test sequences, collectingand displaying test results, and generating reports of gathered results.

To those skilled in the art it is understood that this is just oneembodiment and that there are voltage and power analogs of the disclosedmethods and apparatus. For instance, a parallel-connected IPRA withappropriate values may be substituted and used to test voltagetransformers and protect the instrument transformer secondary circuitagainst short circuit rather than open circuit.

What is claimed is:
 1. A method for testing a transformer including awinding, the method comprising operating a controller to generate aswitching signal, supplying the switching signal to a solid state switchin circuit with a known test load, switching the solid state switch toplace the known test load in circuit with the transformer, and measuringa signal through or across the transformer winding, monitoring zerocrossings of the measured signal, and operating the controller togenerate the switching signal substantially at the zero crossings of themeasured signal.
 2. The method of claim 1 further includinganalog-to-digital converting the measured signal.
 3. The method of claim2 further including locking a phase of the switching signal to a phaseof the measured signal.
 4. The method of claim 1 wherein supplying theswitching signal to the solid state switch in circuit with the knowntest load comprises supplying the switching signal to the solid stateswitch in parallel with the known test load.
 5. The method of claim 1wherein supplying the switching signal to the solid state switch incircuit with the known test load comprises supplying the switchingsignal to the solid state switch in series with the known test load. 6.The method of claim 1 wherein supplying the switching signal to thesolid state switch in circuit with the known test load comprisessupplying the switching signal to the solid state switch inseries-parallel with the known test load.
 7. The method of claim 1further including analog-to-digital converting the measured signal. 8.The method of claim 1 wherein supplying the switching signal to a solidstate switch comprises supplying the switching signal to a field effecttransistor.
 9. Apparatus for testing a transformer including a winding,the apparatus comprising a controller for generating a switching signal,a known test load, a solid state switch in circuit with the known testload, the solid state switch having a control port coupled to thecontroller for supplying the switching signal to the solid state switchto place the known test load in circuit with the transformer, and ameasurement device coupled to the known test load for measuring a signalthrough or across the transformer winding, monitoring zero crossings ofthe measured signal, and supplying the switching signal from thecontroller to the solid state switch substantially at the zero crossingsof the measured signal.
 10. The apparatus of claim 9 further includingan analog-to-digital converter coupled to the transformer and to themeasurement device for analog-to-digital converting the measured signal.11. The apparatus of claim 10 wherein the controller is coupled to theknown test load to lock a phase of the switching signal to a phase ofthe measured signal.
 12. The apparatus of claim 9 wherein the solidstate switch is coupled in series circuit with the known test load. 13.The apparatus of claim 9 wherein the solid state switch is coupled inparallel circuit with the known test load.
 14. The apparatus of claim 9comprising multiple solid state switches, the solid state switchescoupled in series-parallel circuit with the known test load.
 15. Theapparatus of claim 9 wherein the solid state switch comprises a fieldeffect transistor.